This invention relates to a vibration generating device.
A portable device such as a portable telephone which functions as the terminal of a mobile communications device can be made to notify the user of an incoming call by emitting a ringing tone. An alternative, however, is to house a vibrator within the portable device itself or within an accessory thereof and cause the vibrator to vibrate so as to make the user sense the incoming call physically.
A vibration generating device of this kind according to the prior art is as shown in FIG. 4. The device includes a small pencil-shaped motor 80 having a rotary shaft 81 to which a weight 83 is eccentrically attached. Rotating the weight 83 causes the device to vibrate.
A vibration generating device having components mounted more densely than in the device described above is illustrated in FIG. 5. This device includes a permanent magnet 91 arranged in the form of a ring, three coils 97 freely rotatably mounted on a shaft 93 above the permanent magnet 91, and a weight 98 eccentrically attached to the coils 97. Rotating the coils 97 causes the device to vibrate.
In both of the vibration generating devices described above, the method of generating vibration relies upon resonance of the case produced by transmitting to the case the vibration of the rotating shaft caused by the eccentric vibration of a motor. As a consequence, the bearing portion of the motor shaft is acted upon by an excessively large force, which gives rise to problems in terms of the durability and reliability of the vibration generating device.
Since the motor must rotate constantly while vibration is being produced, a current must be passed through the motor during vibration. The result is increased consumption of the battery serving as the power supply of the vibration generating device.
Further, the smaller the motor is made, the less vibrational energy produced. This makes it necessary to enlarge the current flow. This, however, is accompanied by the evolution of heat.